This invention relates to an improved immersion or submerged nozzle for continuous casting.
Conventional immersion nozzles, sometimes referred to as "subentry nozzles", for continuous casting are made of alumina-graphite or zirconia-graphite refractories having good corrosion resistance. However, in the conventional immersion nozzle, some molten metal is apt to deposit on the inner surface of the conduit extending therethrough. In case of aluminum killed steel or the like, deposition of aluminum oxide sometimes blocks the conduit in the nozzle to such a degree that casting stops.
Blockage problems due to metal deposition can be avoided with adequate preheating and/or thermal insulation. On the other hand, in order to solve blockage problems due to aluminum oxide deposition, a porous refractory is used on the inner surface of a conduit extending through a slit type immersion nozzle to allow for introduction of an inert gas into the nozzle. However, the slits cannot be positioned near the molten metal exit of the nozzle. Thus, the blockage problems are apt to occur near the exit of the nozzle. Further, as casting is continued over a prolonged period, carbon in the porous refractory is consumed by oxidation while SiO.sub.2 reacts in a C-CO reducing atmosphere to form SiO which diffuses so that permeability increases. As a result, it is difficult to control the volumetric flow rate of inert gas. Eventually, the inert gas flow rate will increase to where pinholes are formed in the steel product.